Mechanisms and machine components requiring screw threads have a long history in technology. Specifically, the application of screw threads as fastener components dominates over all other means to join parts into assemblies. Although there are many ways to generate screw threads both internal as well as external, experience has shown that taps are the favored means to generate the internal screw thread. There currently exist two tapping methods to generate internal screw threads. The dominant tapping method is by cutting and removing material from the walls of a hole to produce a helical V shaped screw thread. Alternatively, internal screw threads can be created by displacing material to form an internal screw thread. However, tapping by cutting material is generally favored since this method requires lower torque and produces a more perfect thread form.
The dimensional accuracy of the shape and size of the internal screw thread controls the precision and fit of the screw thread assembly. Additionally, the speed of tapping affects the cost to produce an internal screw thread.
There are two materials used to manufacture cutting taps. High-speed steel is widely used for taps because of its high strength. However cemented tungsten carbide is favored as a material for manufacturing other cutting tools over high-speed steel owing to properties such as higher hardness and high temperature stability including the ability to retain hardness at high temperatures. Unfortunately tungsten carbide has lower strength than high-speed steel and is more prone to chipping. The use of tungsten carbide for taps can be expanded by making taps more resistant to chipping through improved tool geometries.
Unlike other cutting tools, taps must reverse rotational direction for removal from the hole after completing a thread. Generally, tapped holes have two basic configurations. Referred to as a through hole, the predrilled hole extends through the thickness of the part. In the case of a through hole, the tap's chamfered cutting section can extend past the bottom of the part and is no longer engaged in the part when reversed. The other situation, referred to as a blind hole, exists where the depth of predrilled hole terminates before reaching the thickness of the part. When tapping a blind hole, the tap can only thread a portion of the drilled hole's depth and therefore the tap is still cutting a thread when it completes the required threading depth and reverses. This situation results in high stresses imposed on the chamfered cutting edges and chipping.
Cutting taps of the current art have a propensity to chip especially when they are still engaged in the work when they reverse, such as when tapping a blind hole.
FIGS. 7A-C show the problem associated with taps of the current art when the tap reverses when still engaged with the workpiece. Rotating for illustration in the counterclockwise direction, the tap generates a chip at the cutting edge (FIG. 7A). The form of the chip depends on the work material. Ductile materials tend to form a long chip like shown. At the moment the tap reaches the required depth, it stops and reverses. The recently created chips are attached to the wall of the hole. Now, rotating in the clockwise direction, the tap's chamfer must ride over the chips attached to hole. Owing to the space formed by the chamfer relief, the chip can become wedged between chamfer surface and the wall of the hole (FIG. 7B). As the tap further rotates in reverse, the wedged chip creates high pressure on the cutting edge, causing the cutting edge to chip (FIG. 7C).
Depending on ductility and hardness, tapped work materials produce different chip shapes. The tap flute helix is varied according to the chip. For example, straight flute taps are used with materials like iron that produce very short chips. Straight flute taps generally cannot be used for tapping blind holes in ductile materials. Since the chips are long, they build up and bind in the flutes causing high tapping torque and frequently breakage. Spiral fluted taps are used in blind holes in ductile materials since the angle of the helix forces the chip out of the hole as the tap advances.
The remedy currently employed by prior art taps is to reduce the amount of chamfer relief on taps used in blind holes. But as previously mentioned, this will cause increased rubbing between tap and workpiece and shorten the life of tap.